High-power laser systems are utilized for a host of different applications, such as welding, cutting, drilling, and materials processing. Such laser systems typically include a laser emitter, the laser light from which is coupled into an optical fiber (or simply a “fiber”), and an optical system that focuses the laser light from the fiber onto the workpiece to be processed. The optical system is typically engineered to produce the highest-quality laser beam, or, equivalently, the beam with the lowest beam parameter product (BPP). The BPP is the product of the laser beam's divergence angle (half-angle) and the radius of the beam at its narrowest point (i.e., the beam waist, the minimum spot size). The BPP quantifies the quality of the laser beam and how well it can be focused to a small spot, and is typically expressed in units of millimeter-milliradians (mm-mrad). (BPP values disclosed herein are in units of mm-mrad unless otherwise indicated.) A Gaussian beam has the lowest possible BPP, given by the wavelength of the laser light divided by pi. The ratio of the BPP of an actual beam to that of an ideal Gaussian beam at the same wavelength is denoted M2, which is a wavelength-independent measure of beam quality.
Laser emitters in high-power laser systems are often solid-state (e.g., semiconductor-based) and may be configured for the emission of multiple beams and operation at high currents. Such emitters, e.g., diode bars and stacks, may therefore generate large amounts of heat during operation, and this heat must be channeled away from the emitters for optimum performance and to prevent emitter damage. Many different types of coolers and heat sinks have been developed to assist the cooling of high-power laser devices. For example, copper-based microchannel coolers and related heat sinks (including micro-impingement coolers) have very low thermal resistance for minimizing the junction temperature of diode and semiconductor laser emitters and laser bars. Thermal resistance values for state-of-the-art active heat sinks are as low as 0.02 K-cm2/W, allowing for over 300 W continuous wave (CW) output power from diode laser bars.
The primary drawback of these active (i.e., utilizing directed fluid flow) heat sinks is limited reliability and lifetime. State-of-the-art active heat sinks have lifetime in the range of 10,000 to 20,000 hours of continuous operation. One of the key limitations of heat sink lifetime is erosion-corrosion resistance. Under typical water pressure and flow-rate conditions, standard copper microchannel coolers tend to show erosion and corrosion in their internal micro-structure. Thus, there is a need for techniques and configurations that enhance the reliability and lifetime of heat sinks for laser emitters and laser bars.